Problem: Luis is 3 times as old as Tiffany and is also 10 years older than Tiffany. How old is Luis?
Explanation: We can use the given information to write down two equations that describe the ages of Luis and Tiffany. Let Luis's current age be $l$ and Tiffany's current age be $t$ $l = 3t$ $l = t + 10$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $l$ is to solve the second equation for $t$ and substitute that value into the first equation. Solving our second equation for $t$ , we get: $t = l - 10$ . Substituting this into our first equation, we get the equation: $l = 3$ $(l - 10)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $l = 3l - 30$ Solving for $l$ , we get: $2 l = 30$ $l = 15$.